I would say get the XD simply because you already have glocks. Variety is the spice of life. Or so they say. It's great that you are familiar with glocks. What happens if you end up in a situation where you do not have access to a glock. Will you be as proficient if you have never practiced with another weapon? Probably not.
I'll answer my own question - Glock.
Shorter trigger travel too.
Translates into the mechanical capability for faster follow-up shots (subject to the skill of the user).
I've handled the 21SF and the 21 and didn't feel much difference between them. I have shot the 21 and found I liked it a lot. I've never shot an XD, but I'm sold on Glocks.
I dont like the grip angle of the glock. Never have never will. I have bought a 21 and a couple other models and ended up selling them. BUT I also am/was a diehard 1911 guy and the XD has the right feel to it. I love mine and you can get great parts from Canyon Creek in ILL. And I even bought a off the rack semicustom XD45 Rich builds.
Both are equally good at function and reliability. It comes downt to what fits your hand and you shoot better.
I just got rid of my last 1911 and am strictly in the XD camp for the duration.
You can ask as many people on the forum as you want which they prefer. It probably won't get you any closer to your decision. IMO the most important features are fit and function. Pick it up. How does it feel in your hand? Fire it. How does it fire for you? Is the point of aim and point of impact good for you? There are your decision factors. If you are into aesthetics (looks), then add that to the list. Personally, the Glock 21 was to big and blocky for me. It did not pass the "Fit" test, so it never made it to the "Fire" test. Before you make a choice, handle and/or fire both.